1. Field of the Invention
The present invention generally relates to an optical article, especially an ophthalmic lens, having a fringe-free monolayer hardcoat on acrylic substrate, as well as to a method for making such an optical article.
In particular, the present invention relates to an optical article wherein the fringe-free monolayer hardcoat on acrylic substrate is prepared by using a specific dendrimer.
2. Description of Related Art
Acrylic polymers are widely used as substrate for optical devices. This kind of substrate are generally based on polymerized (meth)acrylic acid and esters thereof and presents typically a refractive index of around 1.6 at a reference wavelength of 550 nm and expressed at 25° C.
It is known to protect against abrasion and scratches the surfaces of (meth)acrylic polymers-based substrate, by means of hard coatings (abrasion-resistant and/or scratch-resistant coatings) which are typically based on a polysiloxane.
However, it is always difficult to obtain a good adhesion between the (meth)acrylic polymers-based substrate and the abrasion-resistant hard coating.
To enhance adhesion between the substrate and the hard coat, it has been suggested to use a hard coat comprising at least two layers: a first layer, known as primer layer, intended to be deposited on the substrate and a second layer having abrasion-resistant and/or scratch-resistant properties, which is coated on this first primer layer. Hence, the primer layer provides improved adhesion of the hard coat on the substrate.
It should therefore be appreciated to provide a new hard coat composition, which enables to simplify the manufacturing process (less preparation steps, more cost effective process) of an optical article comprising (meth)acrylic polymer-based substrate coated with a hard coat layer, while allowing a good adhesion with said (meth)acrylic polymers-based substrate.
In addition, it is known that the difference in refractive index between the (meth)acrylic polymers-based substrate and the hard coat layer should be as small as possible to limit apparition of fringes due to light reflection and interferences. Fringes are indeed considered as cosmetic defects on the optical article, especially on eyeglasses.
Therefore, it is also desirable to provide a new hard coat composition which prevents the formation of cosmetic defects on the final optical article. In particular, it is desirable to provide a new hard coat composition having a refractive index which is closer to that of the (meth)acrylic polymers-based substrate.
Although the hard coat compositions having a high refractive index, for instance of about 1.6, described in the prior art provide an acceptable adhesion with (meth)acrylic polymers-based substrate, or less visible fringes, they are, however, not fully satisfactory.
Indeed, a unique hard coat composition having improved adhesion, mechanical properties on (meth)acrylic polymers-based substrate, while preventing fringes formation is highly desirable.